I’ve heard it’s been a tough gardening year this year in North America, with all the rain and interesting temperatures. Shame to hear your plant was a victim of it.
Taking a picture and sharing it with us will make it easier to identify the disease, and this should be done so that you can do research and find out how to prevent its spread to any other plants and possibly save any dying ones.
Being only a novice gardener myself, I would agree with others that you likely have late blight, which is described in pete’s link. Here’s some information about it. You may be able to save it if you remove the infected leaves, I’m not sure if any chemical could help at this point but maybe it’s worth going to a gardening store and asking someone about it.
@jonsblond is correct but it extends past tomatoes. Every non-permanant planted item should be rotated, you should never grow the same things over and over in the same spot, for this will make a great home for the pathogens and bugs who prey on that certain plant and eventually you will be met with a devastating loss. Since many of the bugs do not transfer from veggie to veggie, you rotate the crop so that the old ones die and the new crop’s ones are fostered, then rotate to kill those, etc. This is called crop rotation. There’s an example of a (more professional farm style, but still good to know) 7 stage crop rotation in the book “A Self Sufficient Life and How to Live It”, of which my copy has been lent out. I’ll have to come back and post it once it’s returned.